1. Field of Invention
The invention relates to a method for setting the sizes of blown film tubes and a blown film plant comprising a control device for implementing said method.
2. Description of the Prior Art
Blown film plants have been disclosed in a number of patent applications (e.g. U.S. Pat. No. 5,288,219). Blown film plants are usually provided with cooling or temperature control devices. The purpose of these cooling devices, which usually include fans, is to cool the hot, freshly extruded plastic melt, and the film resulting from the melt after the cooling process.
The film tubes are often cooled both from the outside and from the inside. The internal film cooling involves certain technical problems since the freshly extruded film tubes often form a closed film bubble from which the cooling air must be evacuated.
In modern extrusion plants, the film material transitions from a plastic melt to a still very hot film usually shortly after the melt is extruded from an annular gap intended for this purpose. The location at which the melt is transformed to film is often incorrectly referred to as “frost line”. In reality, this transition point is an annular region of the peripheral surface of the melt tube or film tube. This region has an absolutely significant expansion in the transport direction of the film tube, which expansion can range from a few millimeters to a few dozes of centimeters. In this region, the random macromolecules of the melt assume an at least partly organized or at least more organized crystallite structure. Although plastics do not undergo a prompt and defined phase transition, as is possible in the transition of water to ice, the properties of the film melt change due to this transformation into the still very hot film in a manner that is significant for film production. Thus, this transition decisively reduces the stretchability and increases the stability and resistance of the film material. The transition point mentioned above is referred to hereinafter as “frost area” due to the analogy drawn to the freezing of water.
Due to the change in the stretch behavior and resistance of the film during its passage through the frost area, the calibration baskets (see EP 1 488 910 A1), which are often used in the production of blown films, are mounted above the frost area or the location of the frost area is controlled by means of a variation in the cooling devices or temperature control devices such that the frost area is located below the calibration basket.
If the frost area wanders into the calibration basket, the latter could damage the film melt. It is also probable that the softer melt material forms a “bulge’ before reaching the calibration basket. This means that the diameter of the tube before reaching the calibration basket is larger than that of the calibration basket. Machine operators counteract this likewise highly undesirable state by regulating the fans in order to adapt to the cooling behavior and internal pressure of the film bubble. The difficulties involved in this regulation of the fans and the reduced efficiency of this regulation often lead to a pumping behavior in the region of the “bulge” located upstream of the calibration basket. This pumping behavior results in considerable damage to the film or even a tear of the film bubble.
If a frost area is located too far ahead or excessively below the calibration basket, the film may not be stretched to the desired extent and thus may not achieve the desired size. Even in blown film plants without a calibration basket, the adjustment of the location of the frost area is of great significance.
The adjustment of the frost area during size changes, which are performed for completing a new order, poses a special challenge. The amounts of size settings in this connection are particularly large and exceed by several times the changes undertaken in regulating procedures during the processing.
In the case of a size change, in which important physical values change significantly between the initial size and the final size, it is very difficult to optimize the location of the frost area. The word “optimize” in this context is meant to connote that after a size change, the frost area can assume either a different position or the same position as the target position in the transport direction of the film tube. Often the target position of the frost area changes from one order to the other, thereby also necessitating a change in the position of the calibration basket.